![[IMAGE]](contacts/fig45-5.gif)
Fig. 45 represents a line with four stations, A, B, C and D. Suppose the wire broken at F. By connecting the ground wires at B and C, as shown, two distinct circuits are formed. A can work with B, and C with D, showing that the fault is between B and C.
Disconnection is usually caused by the breaking of the line wire, or by a key carelessly left open. Some other causes are wires loose in their binding screws, or defective switches, or the fine wire in or about the relay may be broken. The latter is sometimes burned in two by atmospheric electricity.
117. Partial Disconnection.---
It is rather difficult to discover this fault by the
ordinary relay tests. It is frequently of an intermittent
character, and requires to be very carefully tested for. In the
latter case, the best plan is to cross connect, or interchange
the defective wire with a good one at the terminal, and also one
other station, as in Fig. 46. Suppose the fault is at F, on No.
2 wire ; by cross connection at A and B, as shown, the fault
will shift to No. 1 circuit, showing that it is between those
points. If it were beyond B, it would remain on No. 2 circuit.
In this case, let the wires be put straight at B, and cross
connected at C, and so on, station by station. When the fault
is passed it shifts to the other circuit, and will therefore be
found between the two last stations.
118. To Test for an Escape.---
Call the stations up in rotation, beginning with the one
farthest off, and have them open key for a minute or two. When
a station beyond the escape is open, more or less current will
still pass out to the line through the relay, returning through
the ground from the fault.
Suppose A (Fig. 47) is testing. When the circuit is open at C
or D a current will pass from E through the fault, F, which will
be interrupted when B opens, showing the fault is between B and
C.
119. Testing for Grounds.--- A
ground is tested for in a similar manner. The operator at a way
station can ascertain which side of him the ground is situated,
from the fact that it cuts off or greatly weakens the main
battery current from that direction, when tested with a ground
wire by means of the finger or tongue.
Telegraph lines are very liable to be grounded by the action of
atmospheric electricity upon the lightning arresters. These
should be carefully looked after at frequent intervals,
especially where exposed to dampness, as in cable boxes.
120. Testing for Crosses.--- In
case a cross is suspected between two wires, say Nos. 1 and 2,
instruct the most distant station to open one wire, preferably
the through wire, or No. 1, and ``send dots'' upon the other.
Open No. 2 at your own station, and if the dots sent on No. 2 at
the distant station are received on No. 1, the wires are
crossed. Care must, of course, be taken not to be deceived by
the leakage from one wire to another, caused by defective
insulation. If the wires are in actual contact, the signals
received upon No. 2 wire will be nearly or quite as strong as if
received upon No. 1.
Next, instruct the distant station to leave No. 1 open, and open
it also at your own station. No. 2 will now be free from
interference, and the stations upon it may be signalled without
difficulty. Call them in regular succession, commencing at the
farthest end of the line, and instruct each one in turn to send
dots on No. 2. If the dots are received on both wires the cross
is between you and the station sending ; but if upon No. 2 only,
it is beyond that station. It is better that each operator,
while sending dots, should open the other wire, if
practicable.
The principle of this test will be understood by reference to
Figs. 48 and 49, which represent a two wire line, with four
stations, A, B, C and D, the wires being ``crossed'' between B
and C. The operator testing for the cross is supposed to be at
A. In Fig. 48 station C has No. 1 open, and station A has No. 2
open. If C sends dots on No. 2 the circuit will shift to No. 1,
at the cross, as shown by the arrows, and the dots will come on
No. 1 instrument at A, showing that the cross is between A and
C. In case C were unable to open No. 1 the effect would
evidently be the same, provided it remains open at D.
Now let C close both wires, and B open No. 1, and write dots on
2 (Fig. 49). If No. 2 be open at A, B will be unable to work in
this case, as both wires are open, one at A and the other at B.
With both wires closed at A, B's dots will come on No. 2, the
current from F passing over both wires to the cross, and from
thence on No. 2, No. 1 being open, as shown in the figure. Thus
the fault is located between B and C.
In large offices, where there are a considerable number of
wires, it will often be found a much more convenient and
expeditious method of testing for crosses for the operator to
station himself at the switch with a single instrument, which
can be placed at pleasure on any wire, for the purpose of
communicating with different stations. When any station is
``sending dots'' the testing operator can feel them by placing a
finger upon the ground wire, and another upon the proper line
wire at the switch. The principle involved is of course the
same as in the method first described. In wet weather, however,
testing by the sense of feeling is attended with much
uncertainty, as it is impossible to distinguish between the
effect of a metallic cross, or actual contact, and the leakage
arising from bad insulation.
121. It would be difficult to
specify all the minor interruptions that are liable to occur in
and about telegraph offices, or on the lines ; the operator will
therefore, in many cases, be obliged to depend upon his own
ingenuity for the best method of testing applicable to each
particular case. By carefully studying, however, the principles
heretofore explained, the intelligent telegrapher will usually
be able to cope with any difficulty that may chance to arise in
the ordinary service of the lines.
122. Testing with the Galvanometer and
Resistance Coils.--- In the more accurate and scientific
modes of testing, which have been for some years employed upon
the European lines, the instruments used are the differential
galvanometer and a set of standard resistance coils
(44).
The arrangement of the connections will be understood by
reference to Fig. 50.
The galvanometer coils are wound with two wires of the same
length and resistance, insulated from each other with the utmost
care. The needle is, therefore, surrounded by an equal number
of convolutions of each wire, which are also equi-distant from
it.
The inner end of one coil surrounding the galvanometer,
g, is joined to the outer end of the other at a,
and a key, K, attached to this junction, when depressed, forms
the connection with the testing battery, E. The other ends of
the coils run to binding screws, M M', for the convenient
attachment of lines to be tested. The principle upon which the
action of the instrument depends is the following:
When the battery is connected by depressing the key the current
divides into two equal portions at a, one flowing from
a to M, tending to deflect the needle to the left, and
the other from a to M', tending to deflect it to the
right ; but as long as the two currents are of the same strength
they balance each other, and the needle remains at rest.
Suppose that the terminal M' is connected to a telegraph line
whose remote end is to ground, and the terminal M is connected
through the resistance coils, R, to the ground likewise, as
shown in Fig. 50. We have already seen
(102) that the strength of an
electric current is in all cases equal to the electro-motive
force divided by the resistance. Therefore, if in this case we
let
123. The resistance coils, R,
accompanying the galvanometer, are so arranged as to be
adjustable to any required resistance, from 1 ohm up to 10,000.
For the measurement of still higher resistances one coil of the
galvanometer is provided with three ``shunts,'' or branch
circuits, x, y, z, having resistances
respectively equal to g/9, g/99, and g/999;
therefore, if x be connected, 1/10 of the current will
pass through g, and 9/10 through one wire of the
galvanometer, when connected, and by this means the instrument
may be made to measure any resistance, from 0.001 up to
10,000,000 ohms.
124. Testing for the Distance of
Faults.--- The principle upon which the methods of
distance testing are founded is that of finding the
resistance of the line wire between the testing station and the
fault by means of the apparatus described. When the line is
broken at any point one of the following four cases generally
occurs :
1. Line wire broken, giving full, or nearly full, ground
connection.
2. Line wire unbroken, but gives nearly enough escape to ground
to make signals imperceptible.
3. Line wire broken, without making contact with earth.
4. A cross between two wires, so that signals sent on one are
communicated to both.
125. It is very essential that
the resistance of each circuit should be frequently measured and
recorded, so that when a fault occurs the actual resistance per
mile of the line may be known. If the broken line gives a full
ground, its resistance divided by the resistance per mile at
once gives the distance of the break from the testing station ;
and if the distant station obtains a corresponding result, the
confirmation is complete. Thus, in a line of 100 miles in
length, if the tests from the two extremities indicate distances
of 45 and 55 miles, respectively, the locality of the
interruption is clearly indicated. As the fault, however,
usually gives a very considerable resistance at the point where
the line is in contact with the earth, and the sum of the two
resistances, measured from stations at the opposite ends of the
line, greatly exceeds the resistance of the line itself when
perfect, it is usual in such cases to estimate the fault midway
between the two points indicated. Thus, when the respective
resistances indicate 86 and 26 miles, the sum of these exceeds
100 miles by 12, and therefore half this excess, or 6, is
deducted from each of the measures.
126. When the line is unbroken,
but shows a heavy escape or partial ground, sufficient to weaken
signals, two or three different methods are available for
determining its locality. The first plan is that of direct
measurement, alternately from each end, the distant end at the
same time being insulated, or, in other words, left open, in the
manner explained in the last paragraph. In this case the
resistance of the fault is measured twice over, and is roughly
allowed for by the method of calculation above given.
127. The Loop Test.--- A second
and more accurate method, which gives a measure entirely
independent of the resistance of the fault itself, is known as
the loop test. It is only available, however, in cases where
there are two or more parallel wires on the same route. In order
to make this test, let the operator proceed as follows :
Make the length to be tested as short as possible, and have all
the instruments in circuit taken out. Select a good wire,
similar, if possible, to the one it is required to test. Both
these wires must then be connected together in a loop, at the
nearest available station beyond the fault, without
ground connection. The resistance of the faulty wire, when
perfect, must be ascertained. This may be taken from previous
records, or it may be found thus :
Connect one end of the loop to the + pole of the battery E, and
the other to one of the galvanometer wires at M'. Connect also
the + pole of the battery with the resistance coils, R, at N,
and the -pole of the battery to the key, K, and common terminal,
a, of the galvanometer. Connect the remaining
galvanometer wire with the resistance coils at M (Fig. 51).
Having ascertained the resistance of the loop, arrange the
connections as shown in Fig. 52.
Upon depressing the key, K, the battery current will flow
through M and R, and also through the loop. The resistance
required at R to balance the needle will be equal to the sum of
the resistance of the two lines. Although there is a partial
ground at F it will not affect the measurement, as there is no
other ground in circuit.
Connect the + pole of the battery E to ground, and connect the
-pole to the key K. Connect the perfect wire of the loop to one
of the galvanometer wires at M', and the faulty wire to the
other galvanometer wire at M, interposing the resistance coils
R. Then the key K is depressed the current from the battery E
flows into both lines simultaneously, passing to the ground
through the fault at F. By adding resistance at R, so as to
bring the needle to zero, the resistance a N F will be
made equal to a M' F.
The resistance thus added, deducted from the total resistance of
the loop, previously ascertained, and divided by two, is the
resistance of the line between N and M.
Thus, if the resistance of the loop be 1,000 ohms, and 100 ohms
have been added to the defective wire to balance the
needle---
128. Blavier's Formula for Locating an
Escape.--- Where there is but one wire the following
method may be employed. Three tests have to be taken for the
operation, viz :
This process appears complicated but is in reality very simple.
For example, suppose the line 100 units long, and the fault 68
units distant, and the resistance of the fault 96 units, as
shown in Fig. 53
* See note, (sec.) 104.
// end footnote
We shall, however, have obtained these resistances by
measurement and not by calculation. We therefore have :
The distance, x being known, the others are obtained with
ease ; for R -68 gives y, the distance from the opposite
end ; and T -68 gives z, or the resistance of the fault
itself. This test should be taken from both ends of the line,
if possible. In the above calculation the resistance of the
fault is supposed to remain constant during the measurements ;
but as this is not often the case in practice, the average of
several measurements should be taken.
129. To Find the Distance of a
Cross.--- The two wires in contact form a loop,
provided they are clean, and are twisted together, so that the
contact offers no appreciable resistance. In such a case open
both wires at the nearest station beyond, and test the
resistance of the loop. Half this resistance will be the
resistance of the wire between the galvanometer and the fault,
and from this the distance can be calculated, as before
explained (127). All relays in
circuit must be taken out, or the proper allowance made for
their resistance.
As it is difficult to tell with certainty whether the cross
offers resistance of not, it is a better plan to test it as a
ground. Test each wire in turn by the loop method
(127), grounding the wire at
both ends. The wire tested will then make a ground through the
other wire at the point of contact, and the location of the
latter may be readily ascertained.
Second Method.*--- Suppose two wires, A and B, touch one
another at the point F. Connect A to the zinc of the testing
battery, leaving it open at the remote end ; it will then serve
as a battery wire between the battery and the fault (F). Ground
B at the distant end, and connect it to one coil of the
differential galvanometer at the testing station. Put the other
wire of the galvanometer to ground. The current of the battery
will pass along the wire A and divide at F, one portion going to
ground at the distant end of B, and reaching the galvanometer
through the wire connected with the ground, the other portion
returning to the galvanometer through the nearer portion of B.
If the cross is exactly in the centre of B the needle will not
move, as the two currents will balance each other. If one
section of B is longer than the other, the resistance added to
the shorter section to balance the needle will show the
difference in the resistance of the two sections.
// footnote
* Culley's Handbook, 3d edition, page 279
// end footnote
131. In comparing the
insulation of line wires of different lengths, the insulation
per mile must be ascertained, otherwise the longest wire
will appear the worst ; therefore, multiply the insulation test
in ohms by the length of the wire in miles. If the insulation
is uniformly good throughout the circuit tested, the leakage
will increase in direct proportion to the length of the wire,
irrespective of its thickness or conducting power, for the
resistance of the wire is very small in comparison with the
insulators, and need not be taken into account.
The following example from Culley's work will illustrate this.
The figures given are the results of an actual test :
When connected so as to form a continuous wire, open at the
distant end, the leakage was still 110. The experiment was
repeated, and extended to other wires, with the same result. In
this case the resistance of the insulators was very great
compared with that of the wire---as much as two million ohms per
mile. But on a wet day three similar wires, whose respective
leakages were 196, 185 and 141, making a total of 552, when
looped in a continuous line, as in the second case above, gave a
test of only 476, the distant portion of the wire being in
reality tested by a current weakened by the leakage in the
nearer portions.
132. Testing for Conductivity
Resistance.--- The metallic resistance of the line wires
should be occasionally tested in sections, in the finest
weather. The resistance should be uniformly in proportion to
the length of the wire. If any section discloses an unusually
high resistance per mile, it is very probable that there are
rusty, unsoldered joints in the line, or that the ground
connections are defective. It is difficult for those who have
not tried it to believe the vast improvement that may be made in
any line in a few days by actual measurement, and an inspection
of the sections which give indications of being defective.
It is not an uncommon occurrence to find that a single
unsoldered joint in galvanized iron wire, which appears
perfectly firm and sound, will give a resistance, when tested by
the galvanometer, equal to many miles of line. A line
containing many bad joints will frequently work better in wet
than in dry weather, as the moisture increases the conductivity
of the oxide between the wires at the joints.![[IMAGE]](contacts/fig46-5.gif)
![[IMAGE]](contacts/fig47-5.gif)
![[IMAGE]](contacts/fig48-5.gif)
![[IMAGE]](contacts/fig49-5.gif)
![[IMAGE]](contacts/fig50-4.gif)
E = electro-motive force of battery,
_l_ = resistance of line wire,
_g_ = resistance of galvanometer coil,
_r_ = resistance coils in circuit,
the current in the two circuits surrounding the needle will
be
E E
----------- and -----------
_g_ + _l_' _g_ + _r_
If therefore, the resistance coils in circuit be varied until
r = l, the needle will remain unaffected. As the
earth offers no appreciable resistance to the passage of the
current, the resistance of the line l will be accurately
represented by the amount of resistance interposed at r,
in order to bring the needle to zero. The value of the above
equation will obviously not be affected by any change in the
value of E.![[IMAGE]](contacts/fig51-5.gif)
![[IMAGE]](contacts/fig52-5.gif)
1000 - 100
Then ------------ = 450 ohms, the resistance of the wire
2
between the resistance coils R and the fault.
Let M' F = _x_, N F = _y_.
Then _x_ + _y_ = L, the resistance of the loop.
As R + _y_ = _x_, or R + N F = M' F.
L - R
Therefore, y = -------
2
Suppose that the loop of 1,000 ohms measures 120 miles, then by
proportion,
If 1,000 ohms = 120 miles, 450 ohms = 54 miles.
When an instrument or section of small wire is included in the
circuit, allowance must be made for their resistance. It is a
great assistance in these tests to know from previous records
the exact resistance of every section of the line.
Let R = resistance of the line before it was defective. This must
be obtained from previous records.
`` S = resistance of the line when grounded at the distant end.
`` T = resistance of the line when disconnected at the distant end.
Multiply S by S and T by R, and add the products together ;
subtract from this amount T times S, and also R times S.
Subtract the square root of the remainder from S ; the remainder
will give the resistance, x or the distance of the fault
from the testing station.![[IMAGE]](contacts/fig53-5.gif)
Then R = _x_ + _y_ = 68 + 32 = 100
_y_ x _z_ 32 x 96
*S = _x_ + ----------- = 68 + --------- = 92
_y_ + _z_ 32 + 96
T = _x_ + _z_ = 68 + 96 = 164
// footnote
S x S = 92 x 92 = 8464 \
T x R = 164 x 100 = 16400 > 24864
T x S = 164 x 92 = 15088 \
R x S = 100 x 92 = 9200 > 24288
-------
576
And the square root of 576 is 24; which deducted, S = 92, gives
68 as the resistance of x, or the distance of the fault
from the testing station.
Let L = the total resistance of B.
`` _x_ = resistance of the shorter portion.
`` L - _x_ = `` `` longer ``
`` R = resistance added to shorter portion.
L - R
Then _x_ = -------
2
130. Advantages of Testing by
Measurement.--- The testing of lines by actual
measurement lies at the very foundation of all efforts to
improve the working of our telegraphic system. The insulation
resistance of each of the principal circuits should be measured
every morning, and a record of the results kept for reference.
In England the standard of insulation is 1,000,000 ohms per mile
in the worst of weather. Therefore, a line of 200 miles should
not give less than
1,000,000
----------- = 5,000
200
ohms. If it gives less than this the low resistance is due to
defective insulation. The line should, in that case, be tested
in many separate sections, either from the terminal office or by
a visit to each section. If the resistance per mile is the same
for each section, the fault is probably owing to the nature of
the insulation ; but if, as is usually the case, some sections
are very much worse than others, the trouble will be found in
contact with trees, broken insulators, and the like. A visit to
the faulty locality will disclose the cause of the evil.
The wire A had a leakage equal to....... 29
`` B `` `` `` ....... 30
`` C `` `` `` ....... 50
-----
Total leakage........................ 109
The three wires, when connected together at the testing end and
left open at the distant end, gave a combined leakage of 110.
![[IMAGE]](contacts/fig54-5.gif)